Display device

ABSTRACT

A metal electrode also serving as a black matrix is so formed as to cover the periphery of an IT0 pixel electrode. A region where the pixel electrode and the metal electrode coextend also serves as an auxiliary capacitor. Since the auxiliary capacitor can be formed by using a thin insulating film, it can have a large capacitance. By virtue of the structure in which the black matrix also serves as the auxiliary capacitor, it is not necessary to provide an electrode dedicated to the auxiliary capacitor, thereby preventing reduction in aperture ratio. Further, the black matrix can completely shield a source line and a gate line from light.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the structure of a pixel region of anactive matrix display device and, more specifically, to the structure ofa black matrix (BM) and auxiliary capacitors that are connected inparallel to respective pixel electrodes.

Further, the invention generally relates to the structure of a pixelregion of flat panel displays that require a black matrix.

2. Description of the Related Art

Active matrix liquid crystal display devices are known, in which atleast one thin-film transistor is connected to each of a number of pixelelectrodes which are arranged in a matrix form, and charging, anddischarging of each pixel electrode is controlled by the associatedthin-film transistor.

Each pixel electrode constitutes a capacitor together with a counterelectrode that is opposed to the pixel electrode with a liquid crystaltherebetween.

In actual operation, however, since the capacitance of the capacitorconstituted by each pixel electrode portion is not sufficiently large,an auxiliary capacitor is desired. However, when an electrode of theauxiliary capacitor is made of a conductive material such as a metal, itacts as a light-shielding portion in each pixel, resulting in a decreaseof the aperture ratio.

On the other hand, a light-shielding member called “black matrix” isneeded around each pixel electrode.

In general, regions where source lines (wiring lines for supplyingcurrents to the respective source regions of the thin-film transistors)and gate lines (wiring lines for applying signal voltages to therespective gate electrodes of the thin-film transistors) are arranged ina matrix form have an uneven surface. As a result, a rubbing operationon an orientation film can not be performed properly in those regions,so that the orientation of liquid crystal molecules may be disorderedthere. This may cause a phenomenon that light leaks undesirable or,conversely, a desired quantity of light is not transmitted around thepixels. Further, the liquid crystal may be disabled from performing adesired electro-optical operation in the above regions.

If the above phenomenon occurs, a displayed image is blurred around thepixels, to thereby impair clearness of the image as a whole.

There is known a structure for solving the above problems in whichstructure a light-shielding film is so formed as to cover the edges ofthe pixel electrodes. The light-shielding film is called “black matrix”(BM).

U.S. Pat. No. 5,339,151 discloses a structure including a black matrix.In this technique, as shown in FIG. 2C of this patent, a black matrixextending from gate lines is so formed as to overlap with the edges ofpixel electrodes. This structure is adapted to form auxiliary capacitorsin overlapping portions of the black matrix and the pixel electrodes.

However, the technique of this patent has the following two problems.First, since the black matrix extends from the gate lines. Firstshielding is not complete. This is because source lines cannot beoverlapped with the black matrix to avoid crosstalk. Light leakageshould be tolerated in those portions.

Second, since the black matrix occupies the same plane as the gatelines, naturally it cannot perform light shielding for the sate linesthemselves. Further, the black matrix cannot perform light shielding forthe source lines to avoid crosstalk as described above.

With the recent development of digital equipments, influences ofelectromagnetic waves from low-frequency waves to microwaves have cometo cause problems. In an environment where a liquid crystalelectro-optical device is used, there is concern about the influences ofelectromagnetic waves.

Therefore, liquid crystal display devices need to be so constructed asto be insensitive to external electromagnetic waves.

In view of the above, the above structure as described in U.S. Pat. No.5,339,181 in which the source lines and sate lines which transmit imagesignals are exposed to external electromagnetic waves is not preferable,because the source lines and gate lines may serve as an antenna.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a pixel structure of anactive matrix liquid crystal display device which structure enableseffective light shielding by means of a black matrix.

Another object of the invention is to provide a structure capable ofprotecting the entire device from being influenced by externalelectromagnetic waves.

A further object of the invention is to form auxiliary capacitors havinga necessary capacitance without reducing the aperture ratio.

According to a first aspect of the invention, there is provided adisplay device (as shown in FIGS. 1 and 2) comprising:

-   -   source lines 113 and gate lines 110 arranged in a matrix form;    -   at least one pixel electrode 113 disposed in each region that is        enclosed by the source lines 113 and the gate lines 110; and    -   a light shielding electrode 116 so disposed as to cover the        source lines 113 and the gate lines 110,    -   wherein a periphery of the pixel electrode 118 overlaps with the        light shielding electrode 116, and a coextending region 119 or        120 thereof serves as an auxiliary capacitor.

With this configuration, all the components other than the pixelelectrode 118 and a drain electrode 106 of a thin-film transistor 103can be shielded from incident light. In particular, the source lines 113and the gate lines 110 can completely be shielded from the exterior.This prevents an event that external electromagnetic waves are receivedby the source lines 113 and the gate lines 110 to cause an erroneous orfaulty operation of the device.

Auxiliary capacitors can be formed without reducing the aperture ratio.

In the above configuration, the pixel electrode 113 is a transparentconductive film made of ITO, for instance. While one pixel electrode isprovided in each pixel in the basic configuration, the pixel electrodeof each pixel may be divided into a plurality of parts.

The black matrix 116, which is so disposed as to overlap with theperiphery of the pixel electrode 115, is made of titanium or chromium.The black matrix 116 serves as not only the light-shielding film butalso one of the electrodes constituting an auxiliary capacitor.

It is preferable that the metal electrode 116 be so formed as to overlapwith the periphery of the pixel electrode 118 over its entirecircumference. FIGS. 3A-3E are plan views for explaining the locationalrelationship of each element of FIG. 2.

According to another aspect of the invention, there is provided, asexemplified in FIGS. 1 and 2, a display device comprising:

-   -   source lines 113 and sate lines 110 arranged in a matrix form;        at least one pixel electrode 118 disposed in each region that is        enclosed by the source lines 113 and the gate lines 110; and    -   a light-shielding electrode 116 so disposed as to overlap with a        periphery of the pixel electrode 118,    -   wherein the pixel electrode 118 and the light-shielding        electrode 116 constitute a capacitor through an insulating film        117, and wherein the light-shielding electrode 116 exists in a        different layer than the source lines 113 and the gate lines        110.

According to a further-aspect of the invention, there is provided, asexemplified in FIGS. 1 and 2, a display device comprising in an order aswritten from the side of incident light (i.e., from the top of FIG. 1):

-   -   a pixel electrode 118;    -   a light-shielding electrode 116;    -   a source line 113; and    -   a sate line 110,    -   wherein the pixel electrode 118 and the light-shielding        electrode 116 constitute a capacitor 119 or 120 in between.

In this configuration, by disposing the pixel electrode 118 closest tothe incident light side and disposing the light-shielding electrode(black matrix) 116 next to the pixel electrode 118, the underlaid sourceline 113, gate line 110, and thin-film transistor (except for a drainregion 106) can completely be shielded from incident light.

This configuration is very useful for not only light shielding but alsoelimination of influences of external electromagnetic waves.

Further, the capacitors 119 and 120 can be formed without reducing theaperture ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the structure of a pixel region of anactive matrix liquid crystal display device;

FIG. 2 is a top view of the pixel region of FIG. 1; and

FIGS. 3A-3E are plane views for showing the locational relationship ofeach element of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show the structure of an active matrix liquid crystaldisplay device that utilizes the present invention.

FIG. 1 is a sectional view taken along line A-A′ FIG. 2. FIGS. 1 and 2show the structure of a single pixel that constitutes a pixel area (thepixel area consists of many pixels) of an active matrix liquid crystaldisplay device.

FIGS. 1 and 2 show only the structure on the side of a substrate onwhich thin-film transistors are arranged. Actually, there exists anothersubstrate opposed to it. A liquid crystal is interposed between the twosubstrates through a gap of about several micrometers.

In FIGS. 1 and 2 reference numeral 103 denotes a thin-film transistor,and 101 denotes a glass substrate. Alternatively, a quartz substrate maybe used. Reference numeral 102 denotes a silicon oxide film as anundercoat film. An active layer of the thin-film transistor 103 consistsof a source region 104, offset gate regions 107 and 108, a channelforming region 105, and a drain region 106. The active layer is made ofa crystalline silicon film that has been crystallized by heating anamorphous silicon film or illuminating it with laser light.

Reference numeral 109 denotes a silicon oxide film including a gateinsulating film, and 110 denotes a gate electrode mainly made ofaluminum and extending from a sate line. In FIG. 1, numeral 110 denotesboth of the gate electrode and the gate line.

Reference numeral 111 denotes an anodic oxide film that has been formedby anodization using the aluminum gate electrode 110 as the anode.

Reference numeral 112 denotes a first interlayer insulating film that isa silicon oxide film. Numeral 113 denotes a lead-out electrode connectedto the source region 104 and extending from a source line. In FIG. 1,numeral 113 denotes both of the source electrode and the source line.

Reference numeral 115 denotes a lead-out line connected to the drainelectrode 106 as well as to an IT0 electrode 118 serving as a pixelelectrode. Reference numerals 114 and 117 denote second and thirdinterlayer insulating films, respectively.

Reference numeral 116 denotes a titanium electrode also serving as ablack matrix. Alternatively, a chromium film or the like may be used. Toallow the titanium film 116 to serve as the black matrix, it is soformed as to overlap with the periphery of the pixel electrode 118.

Auxiliary capacitors are formed in regions 119 and 120 where thetitanium electrode 116 and the pixel electrode 118 coextend with aportion of the third interlayer insulating film 117 interposed inbetween. These capacitors can have a large capacitance because theinsulating film 117 can be made thin.

Since the titanium electrode 116 shields also the gate line 110 and thesource line 113 from light, it can prevent charge generation andaccumulation there due to illumination with strong light. The titaniumelectrode 116 also serves as a shield against external electromagneticwaves: it has a function of preventing the gate line 110 and the sourceline 113 from acting as an antenna, to thereby prevent the device fromreceiving undesired signals.

The titanium electrode 116 is so formed as to also cover the thin-filmtransistor 103. This is to prevent an event that light irradiating thethin-film transistor 103 affects its operation. Further, although notshown in the drawings, the electrode 116 is set to an electrically samepotential as a counter electrode.

Although the insulating film 117 is a single layer in the aboveembodiment, it may have a multi-layer structure.

As described above, by overlapping the black matrix and the periphery ofthe pixel electrode through the insulating film, the coextending portionserves as the auxiliary capacitor. This structure prevents reduction inpixel aperture ratio. Further, since the insulating film can be madethin, the auxiliary capacitor can have a large capacitance.

The black matrix can provide effective light shielding as well asprotect the entire device from external electromagnetic waves.

The present invention can be applied to not only black matrix liquidcrystal electro-optical devices but also flat panel displays thatrequire a pixel electrode, a black matrix covering its periphery, and anauxiliary capacitor connected to a thin-film transistor.

1. A display device comprising: a substrate; a plurality of parallelsource lines and a plurality of parallel gate lines formed over saidsubstrate, said source line and gate lines being arranged relative toone another to form a matrix of pixel regions over said substrate witheach of said pixel regions bounded by two adjacent source lines and twoadjacent gate lines; a plurality of thin film transistors formed on saidsubstrate, at least one thin film transistor disposed at eachintersection of said source lines and gate lines in each of said pixelregions; a first interlayer insulating film formed over said thin filmtransistors; a black matrix comprising a light shielding electrodeformed on said first interlayer insulating film and configured to have amatrix pattern with respect to said matrix of pixel regions, said lightshielding electrode disposed to cover completely said source lines andsaid gate lines and to leave exposed a portion of each of said pixelregions; a second interlayer insulating film formed on said lightshielding electrode; a pixel electrode disposed in each of said pixelregions on said second interlayer insulating film, wherein a peripheryof said pixel electrode overlaps with said light shielding electrode toform an auxiliary capacitor with said light shielding electrode, saidsecond interlayer insulating film and said pixel electrode; and acounter electrode in electrical communication with said light shieldingelectrode, wherein said light shielding electrode is electricallyconnected to a same potential as the counter electrode. 2-23. (canceled)